Japanese Patent No. 3671755 discloses an intake control device in which the intake port of an internal combustion engine is partitioned into an upper port part and a lower port part by a partition wall. One of these port parts is closed by a tumble control valve to let intake air flow only through the other port part. The flow velocity of intake air is thus increased and a tumble flow (vertical turbulent flow of intake air) is generated in the combustion chamber so as to accelerate homogenization and stratification of air-fuel mixture. The combustion condition is improved and high fuel efficiency and the lower emissions are achieved.
With the increasing tendency toward tighter regulations on fuel efficiency of vehicles, demand for improved combustion conditions of internal combustion engines to enhance fuel efficiency is growing.
In the system with a tumble control valve as described in Japanese Patent No. 3671755, if the tumble flow intensity is increased by increasing the flow velocity of intake air flowing through the open port part, which is referred to as a tumble port part hereinafter, the combustion condition will be further improved and fuel efficiency will be further enhanced.
Generally, as the piston descends during the intake stroke of the internal combustion engine, the intake pressure on the downstream of the tumble control valve decreases and a pressure difference between the intake pressure on the upstream side of the tumble control valve and that on the downstream side is generated. The larger this pressure difference (amount of decrease in intake pressure on the downstream of the tumble control valve relative to the upstream-intake-pressure) is, the higher the flow velocity of intake air in the tumble port part is and the higher the intensity of tumble flow is.
However, the system has the following problem: the tumble port part is always open and the pressure difference between the intake pressure on the upstream of the tumble control valve and that on the downstream thereof cannot be increased sufficiently. It is difficult to increase the flow velocity of intake air flowing through the tumble port part to increase the intensity of tumble flow and, as a consequence, demand for higher fuel efficiency cannot be met sufficiently.
In the system in which a tumble flow is generated in the combustion chamber, the fuel injected from a fuel injector well evaporates during idling after warm-up of the internal combustion engine. The generation of a tumble flow in the combustion chamber is not so effective in improving fuel efficiency and fuel efficiency cannot be improved sufficiently.